Communication system



I Aug. 7, 1945. I F. MASSA COMMUNICATION SYSTEM Filed Nov. 16, 1942 Z SheetS-Sheet 1 MASTER STATION- SUB- STATION INPUT OUTPUT PRESS TO TALK FIE Y1 33 INVENTOR.

FRANK MASQA 7, 1945- F. MASSA 2,381,174

I COMMUNICATION SYSTEM Filed Nov. 16, 1942 2 Sheets-Sheet 2 INVENTOR.

' FRANK MAJTA A RNEY.

Patented Aug. 7, 1945 UNITED STATES PATENT err-ice.

COMMUNICATION mm 7 Application November is, 1042, Serial No. 485,060

zzomm.

This invention relates to communication sy tems and. more particularly, to telephone systems of the. i'nter-oiiiee, two-way loud speaking type.

The primary object of the invention is to improve the eiiiciency and lessen the cost of interoiiice telephone systems of the type wherein a single transducer is utilized both as a transmitter or as a receiver. Such systems are exemplitied, for example, by 'theUnited States patent to Marcus Glaser. No. 2,087,027.

Another object is to provide a system of the type described that shall enable the satisfactory utilisationof piezoelectric transducers in lieuof 15 with .the construction of the horns. For applitransdueers'of the dynamic type.

Another object is to provide a system of the described that shall'have a substantially iiat over-all response throughout most of the voice-frequency range.

Another-and more specific object is to provide an improved piezoelectric transducer adapted for utilization in telephone systems of the type described.

A telephone system constructed according to the invention comprises at least two transducers of the piesoelectlic type, interconnected through an amplifying device. together with manually operable switching .means whereby each transducer may be caused to. function either as a microphone or as a loud-speaker.

Each transducer comprises a relatively small vibratile diaphragm mechanically coupled to a piezoelectric crystal unit, which unit, for example may be a "Bimorph of the type shown in the United States patents'to A. L. W. Wiiliams No. 2,278,986 and 2,269,498. The sensitivity of the transducer, when functioning as a microphone. is raised by acoustically coupling it to a horn which, preferably, is

of the exponential type and which may be foldeo-eiEBuea to economize space. Above .the' cut- 08 frequency. of, the horn, determined by its rate offlare, the response of the crystal unit, as a microphone remainsreasonably flat throughout the voice. range. As a loud-speaker, however. the piezoelectric crystal unit has a rising frequency response characteristic with constant applied voltage. In order, therefore, to obtain flat over-all frequency response, in a' system comprisms at least two transducers. above the cutoi! frequency of the transducer when utilized as a microphone, compensation is so introduced that the voltage across the input terminals of the transducer functioning as a loud-speaker is inversely proportional to frequency, assuming constant sound pressure at the microphone. Such compensation is obtained by employing a pentode or the equivalent as the single amplifying device between the two transducers, or by utilizing a multi-tube' amplifier the last stage of which has very high output impedance, which provides constant current to the transducer being utilized as a loud-speaker. the crystal unit. the reactance of which is pre- 10 dominantely capacitive below its own resonance frequency, decreases with increasing frequency andv the output from the speaker remains substantially flat. A

' Another phase of the invention is concerned cations such as inter-oiiice communication, it is desirable, of course, that each transmitter-receiver shall be assmali and 'as inconspicuous as possible. However, if the size of the enclosing cabinet is kept small it is obvious that the diam-' cies. The occurrence of such reflections causes peaks and dips in the frequency response characteristic, theirequencies at which the resonances 7 occur being determined by the size of the mouth and the rate of flare of the horn. If identical horns are utilized, at the master and substations,

the peaks are acemtuated but it has been found that relatively smooth response may be obtained by providing the master station with a transducer having a low-frequency cutoff frequency which differs from the cutoif frequency of the substation. The actual design of the horns, with respect to rate of flare. to provide differing cutoff frequencies, is fully explained in the United States patent to n. c. Harrison, No. 1,730,425, and is ramminto those skilledin the art.

In applications where each station is intended to communicate with any one of a number of different stations, reasonably smooth response may 4 be obtained by coupling two horns to each transso the invention are setforth with particularity in the appended claims. The invention itself, however, both as to its organization and its method of operation, together with additional objects and advantages thereof, will best be understood from u the following description of certain specific em- Accordingly, the voltage across tential supply, if desired, in any conventional manner.

The system, as will be clear from the foregoing and-from an inspection of the drawings, is normally in such condition that the master station can call and communicate with any one of perspective of a dual horn constructed according v vention, and Figure 7 is a sectional view of a horn, taken along a line corresponding to the line VII-VII in Figure 6.

In all figures of the drawings, identical elements are similarly designated.

Referring now to Figure 1 of the drawings, an interphone system embodying the invention may comprise a master station I and a plurality of substations, one of the latter being indicated generally by the numeral 3. At each of the substations is located a transducer comprising a piezoelectric crystal element, or Bimorph-5, a relatively small diaphragm l mechanically coupled thereto and a horn 9 having exponential flare acoustically coupled to the diaphragm. For the of the re-entrant type, as indicated in Figure 1, or it may be coiled or folded in any well-known manner.

The transducer i at the master station may be of the same general type as that at each of the substations or it may differ therefrom insofar as the horn thereof is concerned, as will be explained in more detail hereinafter.

One transducer terminal II at the master station may be provided with a ground connection, 'the other terminal i 3 being connected to the movpurpose of conserving space, the horn may be able arm of a multi-contact selector switch, indicated generally by the numeral l5, from the contacts of which, respectively, conductors I! extend to the various substations.

At each substation is disposed an amplifier tube IQ of the pentode type, or its equivalent, a cathode terminal 2| of which is permanently connected to one electrode I I of the substation transducer and is provided with a ground connection. A press-to talk'" switch 23 is incorporated into the substation apparatus which switch, in its normal. or up" position establishes a connection 15, including a blocking capacitor 21, between the anode of the amplifier tube and the other terminal l3 of the substation piezoelectrictransducer element. In the down, or talk position of the substation switch, a connection 29, extending between one contact 'momentarilyengaged by the arm of the selector switch at the master station and the grid of the amplifier tube, is interrupted and a-circuit is established between the said selector switchcontact and the anode of the tube.

In the drawings, the anode potential supply is shown as a battery "and no cathode heating potential source is shown. These potentials, may be derived from any commercial alternating P the substations by manipulating the selector switch while, at the called substation, itis necessary to depress the switch thereat in order to respond to the call.

A simplified embodiment of the invention is shown in Figure 2, suitable for use in situations where two-way communication between but two stations is necessary. The simplified system may include a dual transducer 33 at one station and a similar transducer at the other, the piezoelectric units at the said stations being normally connected in parallel across the output circuit .of an amplifier tube, of the pentode type, disposed at either station. A press-to-talk" switch 35 is provided at each station which switch, when depressed, disconnects that station from the output circuit and connects it to the input circuit thereof.

In the two embodiments of the invention thus far described, for purposes of convenience the horn at each station has been described as being of the same simple exponential type, either folded, coiled, straight or re-entrant. As a matter of fact, however, it has been found that although the horn greatly increases the sensitivity of the transducer when utilized as a microphone, the necessarily small size of the mouth opening thereof, in comparison with thewave length of the lowest frequency of voice-reproduction, causes the occurrence of large mouth reflections at the lower frequencies. The presence of reflections gives rise to peaks and dips along the frequency range. the location of the peaks as a function of frequency being determined by the size of the mouth and rate of flare of the horn. For example, such peaks may be represented conventionally by the curve designated 1: in Figure 4. In order to neutralize the peaks, therefore, a transducer horn 31 at one station, as shown in Figure 5, may have a greater rate of flare than the horn 33 at the other station, giving rise, for example to peaks and dips in its response characteristic exemplified by the curve 11 in Figure 4.

When two such horns are utilized in conjunction with each other, the overall response characteristic of the two-station system, at frequencies in the neighborhood of the several cut-off frequencies of the horns, respectively, is smoothed out as indicated by the curve is in Figure-4.

Specific design data for the several horns need not be given herein: they may be determined by consulting the Harrison patent or the treatise by H. F. Olson and Frank Massa, Applied acoustics, published by Blakiston.

In the event that it is desirable that the master station be, able to communicate with any one of a plurality of substations and that the substations may also communicate among themselves, it has been found desirable to adopt a standard dual exponential-horn-transducer for each station. Such a transducer, exemplified by Figure 3"of the drawings, may be constituted by two horns, I3 and 31, having differing rates of flare, both of which merge into a common throat acoustically, coupled to the diaphragm of the transducer. Such dual horn devices have given satisfaction in smoothing out the overall response characteristic of an interphone system in which they are utilized.

It also lies within the scope of the invention to utilize coaxial exponential type horns, as exemplified by'Flgures 6 and '7 of the drawings. Horns of the last referred to type, as-Well as those of the type exemplified by Figure 3, may also be re-entrant, folded or coiled upon themselves, if desired, for the purpose of economizingfspace.

In the operation of an interphone system constructed according to this invention, the rising frequency response of the transducer, when functioning as a loud-speaker, is so compensated by the characteristic of the pentode'amplifier that the overall response of the system, leaving the horns out of consideration, is relatively flat. By utilizing horns having differing rates of flare, as. explained, the effects of horn-reactance are also substantially overcome and a system is thus provided wherein the intelligibility of speech and the sensitivity are materially better than in anal- ..ogous systems known previous to this invention.

Although certain specific embodiments of the invention have been chosen for purposes of illustration, the inventor is fully aware that numerous modifications thereof will be apparent to those skilled in the art to which it pertains. The invention, therefore, is not to be limited in scope except insofar as is necessitated by the prior art and by the spirit of the appended claims.

What is claimed is: I 1. In an electrical communication system having a given frequency range, a microphone havmg a substantially fiat response characteristic. a

' to give an overall response pattern for the two terized in this: that the amplifier includes an output tube having the characteristics of a pentode.

'7. In combination, a transducer comprising a piezoelectric crystal element, a diaphragm operatively connected to said element, and at least two horns acoustically coupled to said diaphragm, the said horns having different cutofi frequencies.

v8. The invention set forth in claim 7, characterized in this: that the cutoff frequency of each horn is a function of its rate-of flare.

9. The invention set forth in claim '7, characterized in this: that each of the horns approximates in contour and flare the so-called exponential horn.

10. The invention as set forth in claim 4 further characterized in this; that said plurality of horn means are separated in space and are electrically connected together.

11. The invention as set forth in claim 4 further characterized in this; that said plurality, of horn means are co-axial.

12. The invention as set forth in claim 4 further characterized in this; that said plurality of horn means have a common throat opening;

13. The invention as set forth in claim 4 further characterized in this; that said plurality of horn means have a common throat opening and each said horn means is substantiallyrectangular in cross-section normal to the axis thereof.

14. The invention as set forth in claim 2, fur.-

and each of which is adapted to function eitheras a telephonic transmitter having horn cactance peaks and dips or as a receiver having horn reactance peaks and dips each of said horn means of said pair having a different rate of horns which i smoother than the response pat- I tern for either one of said horns alone.

3. The invention setforth in claim 1, charac- 'terized in this:- that the receiver comprises a piezoelectric crystal element the reactance of which is capacitative over most of the voice-frequency range.

4. In combination, a plurality of finite flared horn means each of which has acoustic impedance variations, said plurality of horn means having different 'rates of flare whereby the impedance variations of one of said horn means at least partially neutralizes the impedance variations of other of said plurality of horn means to give an overall response pattern for the plurality of horn means which is smoother than the response pattern for any one of said horn means alone.

5. In an acoustic system, a pair of finite flared horns each of which has acoustic impedance variations in its response pattern, said two horns having exponential flare rates which differ whereby the impedance variations in their response patterns at least partially neutralize each other to give an overall response pattern for the system which is smoother than the response pattern for either one of said horns alone.

6.. The invention set forth in claim 1,. characflare whereby the said horn reactance peaks of one horn tend to cancel the horn reactance dips' ceiver and a microphone each comprising a piezoelectric crystal element, a diaphragm operatively connected to sa d piezoelectric crystal element, and at least two horns each exhibiting horn reactancepeaks and dipsand having different cut off frequencies acoustically coupled to said diaphragm; said receiver having a rising response characteristic and a. capacitive reactance,

' nected to said receiver whereby said system has a substantially flat overall frequency response over most of said given frequency range and is substantially free from pronounced horn reactance peaks.

19. The invention as set forth in claim 18 further characterized in this: that each of said two horns which is acoustically coupled to a diaan electro-mechanical translating device, a diaphragm operatively connected to said translating device, and at least two finite horns acoustically coupled to said diaphragm, the said horns having difierent rates of flare.

22.The invention as set forth in claim 21,

characterized in this: that the size of the mouth opening of eachof said two horns is less than the length of one wave at the lowest frequency of 10 reproduction.

FRANK MASSA. 

